lepton

History

The electron was discovered by J.J. Thomson in 1897. The muon was found by Carl Anderson in 1936 and initially mistaken for a meson. The tau lepton was discovered in 1975 at the Stanford Linear Accelerator. Neutrinos were first detected in 1956. Wolfgang Pauli predicted the existence of neutrinos in 1930.

How it works

Leptons manifest themselves in decays and collisions. The muon is a heavy cousin of the electron, lives for a couple of microseconds, and decays into an electron and neutrinos. Neutrinos oscillate, changing their flavor along the way like a chameleon changing color. All this is governed by the weak interaction, and for charged leptons, also by electromagnetism.

History

The electron was discovered by J.J. Thomson in 1897. The muon was found by Carl Anderson in 1936 and initially mistaken for a meson. The tau lepton was discovered in 1975 at the Stanford Linear Accelerator. Neutrinos were first detected in 1956. Wolfgang Pauli predicted the existence of neutrinos in 1930.

How it works

Leptons manifest themselves in decays and collisions. The muon is a heavy cousin of the electron, lives for a couple of microseconds, and decays into an electron and neutrinos. Neutrinos oscillate, changing their flavor along the way like a chameleon changing color. All this is governed by the weak interaction, and for charged leptons, also by electromagnetism.

History

J.J. Thomson identified the electron as a particle in 1897 by measuring its e/m ratio. The muon was discovered in 1936 in cosmic rays and was first confused with Yukawa's meson. The tau lepton was detected by Martin Perl's group in 1975. The existence of neutrinos was postulated by Wolfgang Pauli in 1930 to save the energy conservation law in beta decay; neutrinos were first captured by Reines and Cowan in 1956.

How it works

Leptons interact through the weak interaction (exchange of W±, Z bosons), and charged ones also electromagnetically (photon). The Dirac equation describes free charged leptons. Neutrino oscillations require nonzero mass and mixing, described by the Pontecorvo-Maki-Nakagawa-Sakata matrix. Charged leptons are stable (electron) or decay via the weak interaction (muon, tau). The Pauli principle forbids two identical leptons from occupying the same quantum state, which determines the structure of atoms and neutron stars.

Open problems & fun facts

Why are there three generations of leptons and what is their mass hierarchy?; Are neutrinos Dirac or Majorana particles (is matter and antimatter identical or not)?; What is the absolute mass scale of neutrinos and does a sterile neutrino exist?; Unification of leptons and quarks in Grand Unified Theories.

💡 In neutrino observatories like IceCube, they use a cubic kilometer of Antarctic ice to catch a flash from a single neutrino colliding with an atom.